In many fields it it is important to know when a specific threshold temperature has been passed by a particular commodity, machine, or the like. For various reasons it is valuable to know if at any time one or more threshold temperatures have been passed by the thing being monitored, especially when it has not been under the control of the person most concerned with this threshold temperature.
Pharmaceutical and blood products as well as organs destined for transplant use are recovered or fabricated at one temperature, stored at another lower temperature, and normally used at ambient temperature. Blood plasma, for instance, is recovered at body temperature which is +37.degree. C., is stored at below -30.degree. C., and is used at about +20.degree. C. Similarly whole blood is recovered and used at the same temperatures, but must be stored at between +2.degree. C. and +6.degree. C., neither hotter nor colder or the blood will be unusable. Whenever in the chain between initial recovery of these products and eventual use these temperature parameters are violated, the result is a serious degradation of the product. As a result the products are extremely expensive and rare, and careful consumers take enormous pains to ensure that the product is good, without ever having absolute surety that temperature limits have not been passed.
Similarly in the frozen-food business it is essential that the foodstuff, for example meat, be frozen rapidly to a very low temperature, and be maintained below a predetermined lower limit. The rapid freezing is needed to prevent excessive formation of cell-damaging crystals, and maintaining the desired cold is necessary to prevent subsequent formation of ice crystals which can puncture cells and leave the foodstuff wet and pulpy. Similarly the shelf life of the product is greatly decreased if it is frozen, thawed, and refrozen. In this particular situation an irreversible temperature witness is needed since the consumer cannot ascertain from looking at a product that is frozen if it will be good when thawed.
Electronic equipment is also temperature sensitive. It is manufactured at one temperature, is normally used at a higher temperature, but should not exceed a yet higher temperature. When the temperature of a computer, for instance, starts exceeding this threshold temperature, some sort of service is normally advisable in order to avert a later breakdown, even though, for instance, all that may be needed is a new disk-drive filter.
A classic temperature witness is described in U.S. Pat. No. 1,535,536 and in French patent No. 1,391,673. A body that is made of a material whose melting point is the desired threshold temperature is juxtaposed with a dye so that when the threshold temperature is passed the body melts and absorbs the dye. The resultant color change remains even if the body is refrozen. The device must be made at low temperature and stored at such temperature, making its use and transport somewhat difficult.
In French patent No. 1,515,914 such an arrangement is proposed where the dye is held in a frangible capsule in the body to provide so-called autoreadying action The capsule keeps the dye away from the body during manufacture and before use. The first time the unit is frozen, however, the capsule ruptures. Since by this time the body and the dye are fairly frozen, there is no mixing of the two until they thaw, whereupon they mix and make an irreversible color change.
The best of such temperature-witnessing devices are not widely used. They are often very difficult and hence expensive to manufacture. Frequently they can be tampered with and reset, and never are they able to respond reliably to several different temperatures.